Light-Independent Reactions and the Calvin Cycle

The light-independent reactions, also known as the Calvin cycle, occur in the stroma of chloroplasts. These reactions use the products of the light-dependent reactions (ATP and NADPH) to fix carbon dioxide into organic compounds.

Definition: The Calvin cycle is a series of biochemical reactions that use ATP and NADPH to convert carbon dioxide into glucose.

The Calvin cycle consists of several steps:

1. Carbon fixation: CO₂ combines with ribulose bisphosphate (RuBP) to form an unstable 6-carbon compound, catalyzed by the enzyme rubisco.

2. Reduction: The unstable compound is split into two 3-carbon molecules called glycerate-3-phosphate (GP).

3. Regeneration: GP is reduced to triose phosphate (TP) using ATP and NADPH.

Highlight: The role of ATP and NADPH in photosynthesis is crucial in this stage, providing the energy and reducing power needed to convert CO₂ into sugar.

4. Synthesis of glucose: Some TP molecules are used to synthesize glucose, while others are recycled to regenerate RuBP.

Example: Out of every six carbon atoms that enter the cycle, one glucose molecule (C₆H₁₂O₆) is produced, while the rest are used to regenerate RuBP.

Light-Dependent Reactions in Photosynthesis

The light-dependent reaction in photosynthesis is a crucial process that occurs in the thylakoid membrane of chloroplasts. This reaction harnesses light energy to produce ATP and NADPH, which are essential for the subsequent light-independent reactions.

Definition: Light-dependent reactions are the initial stage of photosynthesis where light energy is converted into chemical energy in the form of ATP and NADPH.

The process involves several key steps:

1. Photoionization of chlorophyll: Light energy excites electrons in photosystem II (PS2), raising them to a higher energy level.

2. Photolysis of water: The excited electrons from PS2 are replaced by splitting water molecules, releasing oxygen as a byproduct.

Equation: H₂O → O₂ + H⁺ + e⁻

3. Electron Transport Chain (ETC): Excited electrons move down the ETC, losing energy and pumping H⁺ ions into the thylakoid space.

4. Chemiosmosis: The proton gradient created by the ETC drives ATP synthesis through ATP synthase.

Vocabulary: Chemiosmosis is the movement of ions across a selectively permeable membrane, down their electrochemical gradient.

5. NADPH Production: Electrons that have passed through the ETC are energized again in photosystem I (PS1) and used to reduce NADP⁺ to NADPH.

Highlight: The light-dependent reaction steps produce two key products: ATP and NADPH, which are crucial for the Calvin cycle in the light-independent reactions.